The present invention relates to a device having a magnetic circuit which interacts with a yoke member made of soft magnetic material. The yoke member carries a magnetic flux produced by a magnetic-field-generating device and generates a magnetic stray field. This type of magnetic device is described in the German Published Patent Application 37 37 133.
Homogeneous field magnets which generate magnetic basic fields in nuclear spin tomography systems (nuclear magnetic resonance tomography, imaging or spectroscopy) are known. The magnetic field of this type of basic field magnet must be sufficiently homogeneous in an imaging or examination area (useful volume) and must generate a predetermined magnetic induction B.sub.o in that area. For this purpose, normal-conducting coils or permanent magnets are provided as a magnetic-field-generating device. They are mounted, for example, on a C-shaped yoke :member of soft magnetic material. This yoke member carries the generated magnetic flux outside of the useful volume situated between the pole faces of opposite pole shoes.
The magnetic circuit can in fact be substantially closed with this type of yoke member. However, such a yoke member creates a magnetic stray field that is relatively far-reaching. Such stray fields are undesirable, particularly in nuclear spin tomography.
Therefore, the object of the present invention is to provide a magnetic device having the characteristics mentioned at the outset with a reduced stray field produced by the yoke member.